Fix registration of protocol analyzers from inside plugins.

With the recent packet manager work, it broke to register a protocol
analyzer for a specific port from inside a plugin's initialization code.
That's because that registration now depends on the packet manager being
set up, which isn't case at that time a plugin's `InitPostInit()` runs.
This fix contains two parts:

    - Initialize the packet manager before the analyzer manager, so that
      the latter's `InitPostScript()` can rely on the former being
      ready.

    - Change the analyzer manager to (only) record port registrations
      happening before it's fully initialized. Its `InitPostScript()`
      then performs the actual registrations, knowing it can use the
      packet manager now.

This comes with a `cmake/` to add a missing include directory.

testing/btest/plugins/protocol-plugin/src/Plugin.h

@@ -10,6 +10,8 @@ class Plugin : public zeek::plugin::Plugin
 protected:
 	// Overridden from zeek::plugin::Plugin.
 	zeek::plugin::Configuration Configure() override;
+
+	void InitPostScript() override;
 };
 
 extern Plugin plugin;